Many industries still rely on the municipal sewage treatment works as a disposal route for their aqueous effluents. This is usually the most cost effective treatment process, provided that the sewage treatment works can accommodate these waste streams. However some toxic organic substances present in these industrial effluents pass through a conventional sewage treatment works without being sufficiently removed and/or destabilise the sewage treatment system. This has resulted in more and more strict control on industrial waste water discharge, called Trade Effluent Control. The control of toxic materials known as "red list" substances (a term used in the UK equivalent to the "priority pollutants" in the USA) is particularly stringent, and the discharge consents for many of the red list substances are on the order of ppb (parts per billion). The concentrations of such substances in industrial effluents are often on the order of hundreds ppb, compared to on the order of thousands ppm (parts per million) of other organic substances. Current treatment processes such as activated carbon adsorption and oxidation are not selective to the red list substances, which are only degraded after most of the other organic substances have been destroyed.
Current processes for dealing with red list substances normally utilise on-site treatment plant. Since most of the other organic substances are removed to the same order of concentration level when the red list substances content is reduced to the relevant discharge consent level, this is very expensive both in capital investment and operation cost compared with if the municipal sewage treatment works had to treat only those organic substances having discharge consents well above that for the red list substances.
JP-A-50142466 (CA 84(12):79406c) discloses the oxidation with ozone and subsequent dewatering of an iron-containing sludge produced during flocculation of waste water with ferrous sulphate at pH 10.2. Reportedly, the ozonation oxidises the precipitated ferrous hydroxide to ferric oxide and thereby markedly decreases sludge formation compared with conventional treatment with hydrated lime. In particular, when a sludge obtained by treatment of one litre of a dye-containing waste water with ferrous sulphate was oxidised with 90 mg ozone, the volume of sludge formed was 70 cm.sup.3. After dewatering, the volume of the remaining sludge cake was 14 cm.sup.3. This compared with volumes of 210 cm.sup.3 and 21 cm.sup.3, respectively, when the slurry was oxidized with 20% Ca(OH).sub.2.
U.S. Pat. No. 4,049,545 discloses the treatment of waste water by addition of an alkaline coagulant aid to pH 9.0-10.5 and the subsequent addition of precipitating agents in at least two successive steps to lower the pH of the mixture by about pH 1 each step. Exemplified precipitating agents include aluminium and ferric salts. It is stated that, after the last precipitation step, the pH of the water should be between 6 and 8.5. There is no suggestion that selective precipitation should initially be carried out at pH 6 to 8 as required by the present invention. Further, there is no reference in the Application to oxidizing precipitated sludge.
U.S. Pat. No. 4,717,484 discloses a treatment of waste water in which a metal salt is added to form a flocculated mixture of metal hydroxide and organic sludge. After degasification and separation from the treated water, the metal salts in the flocculated mixture are dissolved by increasing pH and recycled as flocculant. Exemplified flocculants are aluminium and iron salts. There is no reference to selected precipitation of particular organic waste by control of pH prior to addition of the flocculant and no reference to oxidation of the precipitated sludge.
Many surface water treatment plants and industrial waste water treatment works employ chemical precipitation reagents such as metal salts, especially Fe or Al salts, to remove organic material by charge neutralisation thereof and co-precipitation/adsorption. These processes generate a large quantity of sludge which usually is settled out from the treated water in a settling tank or floated off the treated water via a dissolved air flotation process. This sludge can then be thickened and dewatered and sent off-site for disposal. This practice involves operational costs for both reagent consumption and sludge disposal. Currently all attempts to reduce cost have been directed towards the reduction in sludge disposal cost, for example by improving sludge dewatering thereby reducing the volume of sludge to be disposed of.